A monitoring device for a battery pack, which includes a plurality of battery cells, has at least one ultrasound source and at least one ultrasound sensor. The ultrasound source can be configured to generate and direct ultrasound at one or more battery cells of the battery pack. The ultrasound sensor can be configured to detect ultrasound reflected from or transmitted through one or more cells of the battery pack. A battery management unit receives one or more signals from the ultrasound sensor responsive to the detected ultrasound. The battery management unit can be configured to determine a state of the battery pack based at least in part on the detected ultrasound.